On account of the problem of CO2 emissions during idling, modern motor vehicles are increasingly being equipped with a start/stop function. In a start/stop function, the motor is turned off, for example when the vehicle stops at a traffic light, in order to save fuel and to reduce CO2 emissions. The motor is restarted by means of the starter in order to continue driving. Since the starter draws current of up to 800 A in order to start the motor, a voltage of up to 6 V is dropped across the internal resistor of the vehicle battery. The available terminal voltage in the electrical system therefore briefly drops to 12 V-6 V=6 V. In addition to the start/stop operation at traffic lights, it is already known to be able to turn off the motor (internal combustion engine) when it is not in use even while the vehicle is moving (“freewheeling motor off”; “start/stop on the move”). Restarting of the motor vehicle is to be ensured in every case.
DE 10 2008 054 885 A1 discloses an apparatus for controlling the supply of power to an electrical system of a vehicle, to which electrical system the load can be connected, comprising a first energy store for providing electrical energy to a starter, a generator for generating a generator voltage and a first controllable switching element which is provided between the first energy store and the generator and which is formed in such a way that it permits current flow only in one direction from the first energy store to the generator during operation of the vehicle. In this case, the switching element can also have a further switching state in which the switching element behaves in the manner of a conductor. The controllable switching element may be a field-effect transistor, in particular a MOSFET. Since field-effect transistors can be switched with low gate voltages and a minimal time delay, it is therefore possible to react quickly to changes in the current flow across the controllable switching element or severe voltage fluctuations in the electrical system. For example, a field-effect transistor or field-effect transistors which are connected in parallel can be arranged such that the forward direction of the intrinsic diode or diodes of the field-effect transistors corresponds to the direction from the first energy store to the generator. This ensures that, when the transistor is turned off, no current can flow from the generator to the first energy store. Secondly, even when the transistor is turned off, some of the current which flows from the first energy store to the electrical system can flow through the intrinsic diodes of the field-effect transistors. The apparatus further comprises a further energy store for providing electrical energy to the loads, the further energy store being arranged in such a way that the first controllable switching element is arranged between firstly the first energy store and secondly the generator and the further energy store. A charging circuit is also provided, wherein the charging circuit is formed, during operation of the vehicle, in such a way that the first energy store is charged to a voltage UE1 which is equal to or greater than the voltage of the second energy store UE2 and is lower than the voltage of the generator. The first energy store may be in the form of a double-layer capacitor and the second energy store may be in the form of a battery. The first energy store provides electrical energy to the starter for a warm start and/or damps voltage fluctuations through the load, wherein the second energy store provides electrical energy for the starter in the event of cold starting.
DE 10 2007 026 164 A1 and DE 103 59 486 A1 likewise disclose motor vehicle electrical systems where the starting energy of a starter is provided by double-layer capacitors.